The military, police and some commercial installations have vehicles that are provided with a virtual forest of antennas to cover various frequency bands. As a result there is a requirement for continuous coverage in a single antenna that operates between from the VHF bands at 30 megahertz all the way up to the 6 UHF gigahertz frequencies.
In order to be able to provide multi-band coverage, up to 4 or 5 antennas are separately utilized on a vehicle. The bands of interest for the military are the 30-88 megahertz band, the 108-156 megahertz band, the 225-450 megahertz band, the 1350-1550 megahertz band and the 1650-1850 megahertz band.
As mentioned above, there is a necessity for military, law enforcement and even commercial vehicles to be equipped with communication devices to permit operators to exchange information with a variety of different information services, command and control and dispatch centers. Also, GPS coverage is often required for geolocation. While these vehicles can employ multiple separate antennas designed to communicate effectively at a particular frequency range, there is a requirement for a single antenna that may be mounted to existing vehicles so that one antenna can have the gain of legacy antennas, while supplanting the forest of antennas previously utilized.
More particularly, a so-called Sincgars antenna typically operates between 30 megahertz and 88 megahertz, where the 30 megahertz legacy antenna has a −3 to −6 db gain over a ¼ wave monopole. The 30 megahertz legacy antenna is typically a monopole antenna whose gain is directly proportional to antenna volume. It is noted that for 30 megahertz, a quarter wavelength is 8 feet which makes a quarter wave antenna unusable in a wide variety of applications.
What is therefore required in addition to multi-band operation is an antenna whose overall height is no more than 4 or 5 feet.
It would therefore be desirable for instance to be able to replace the army AS3900A whip antenna with a single relatively short multi-band whip antenna that could provide the requisite gain.
One antenna capable of multi-band use is described in U.S. patent application Ser. No. 11/641,041 assigned to the assignee hereof. This antenna is designed to operate in the 30 to 88 megahertz band. However it is over 105 inches tall. Another problem with this antenna is that it is fabricated utilizing a number of sections of tubing that are screwed together. It has been found that these antennas are not readily fabricatable and deployable in the field due to the variability when screwing the sections together and due to the fact that from a storage point of view a 105 inch antenna is not practical.
Thus, especially for the Sincgars radio band, providing such an antenna, primarily for voice communications, has its problems. Moreover, when considering vehicle mounted antennas operating above a ground plane, variability in the ground plane configuration causes matching and radiation pattern problems because vehicle configurations can vary significantly. It therefore becomes critical as where on the vehicle to mount the antenna.
While it might be thought that any antenna could be tuned for each vehicle, such antennas are not practical and the simple solution is to simply avoid frequencies where VSWR is high, with the obvious coverage disadvantages.
Moreover, aside from its length and multi-part construction, it was found that standard meander lines used to separate out the bands did not adequately act as traps. Thus, while various dipoles were designed to operate in various bands, the traps did not function properly to switch from a short to a trap at the band demarcations.
Secondly, especially in the middle and upper bands, the prior antenna did not exhibit sufficient gain so that the antenna could not match or exceed legacy antennas.
Further, it was found that in shortening the prior antenna, linearly downsizing the meander lines did not result in the either sufficient gain or sufficiently low VSWR.
Moreover, it was almost impossible to tune the meander lines once in place. The result was that pre-tuned antennas would not exhibit the required tuning when vehicle mounted.
Finally, the antenna could not pass the so-called oak-beam test, in which the antenna is to withstand repeated impact with an oak beam at 30 mph.
For these reasons the antenna design described in the aforementioned patent applications had to be abandoned and a new antenna had to be designed that would solve the problems noted above.
By way of further background, it has been proposed to miniaturize antennas by using so-called meander line loaded antennas exemplified by U.S. Pat. Nos. 5,790,080; 6,323,814; 6,373,440; 6,373,446; 6,480,158; 6,492,953 and 6,404,391, all assigned to the assignee hereof and incorporated herein by reference. While these meander lines have been utilized in the past for impedance matching and tuning purposes, they were not utilized to provide chokes or traps between various dipole segments so as to make a single whip operate in a multi-band mode.